Rotation matrix addition
Euler rotation as a matrix
Would imagine that what you want for x, y, z is an Euler rotation.
The KX_GameObject.localOrientation expects a 3x3 matrix, an Euler or a Quaternion.
euler_rotation = mathutils.Euler((x, y, z), 'XYZ') # XYZ is default
# 3 x 3 rotation matrix
R = euler_rotation.to_matrix()

this is the equivalent of
Rx = mathutils.Matrix.Rotation(x, 3, 'X')
Ry = mathutils.Matrix.Rotation(y, 3, 'Y')
Rz = mathutils.Matrix.Rotation(z, 3, 'Z')
R = Rz * Ry * Rx # ( XYZ order)

Can either set your local orientation to this matrix, or to rotate by it
own.localOrientation = R * own.localOrientation

remember all rotations need to be in radians. x = math.radians(45)
Putting it together in game engine, the following in a text block named rot.py
from mathutils import Euler
from math import radians
x = y = z = radians(5)
R = Euler((x, y, z)).to_matrix()

def main(cont):
    own = cont.owner
    # color vector xyzw  ~ rgba
    own.localOrientation = R * own.localOrientation

Assign to the left mouse click. Now on every click the owner is rotated 5 degrees on each axis each click